Polarographic method and apparatus



POLAROGRAPHIC METHOD AND APPARATUS ,POM KHK/ 770/504 ATTORNEY UnitedStates Patent tice 3,013,758 Patented Jan. 1963 3 073,758 POLAROGRAPHIClt/ETHOD AND APPARATUS Rolf K. Ladisch, 255 Windermere, Lansdowne, Pa.Filed Sept. 4, 1959, Ser. No. 838,131 15 Claims. (Cl. 204-1) The presentinvention relates to an electrochemical method and apparatus in theiield of polarographic analysis. Characteristics as well aspotentialities and li-mitations of polarography, derivativepolarography, differential polarography, oscilloscopio polarography,controlled potential separations, and amperometry have been eX-tensively discussed in .the literature. See for instance ChemischeAnalysen mit dem Polarographen, by H. Hohn, l. Springer Verlag, Berlin,1937; Polarography, by I. M. Kolthofr` and J. I. Lingane, lntersciencePublishers, New York, 1952; Die Polarographie in der Medizin, Biochemieund Pharmazie, by M. Brezina and P. Zuman, Akad. Verlagsges., Leipzig,1956.

These methods are based on the fact that electrolysis of a solutionbetween a polarizable electrode and a nonpolarizable electrode producescurrent-voltage relationships that are typical for the quantity andquality of the investigated test species in the solution. Theoutstanding feature of the classic polarographic electrolysis cell isthe polarizable dropping mercury electrode, i.e., mercury droplets beingdischarged periodically into a solution from a fine bore capillary undera driving head of mercury. This very feature, which has permitted theinitiation of eX- tremely useful polarographic methods in research Work,seems now to militate against a more general use of polarography as acommon analytical method and particularly as a tool for monitoring andcontrolling industrial process streams. The periodic growth and fall ofthe mercury droplets cause oscillations in the current-voltage curveswhich are an obstacle in evaluating such curves. Also an undesiredcondenser current builds up whenever a new mercury droplet is beingformed at the capillary. Furthermore, the size of the droplets isnecessarily small, thus limiting the method to a fraction of thesensitivity that could otherwise be obtained. In addition, formation ofthese tiny droplets is a delicate process and is therefore affected by anumber of incidental factors, such as mechanical vibrations,"slant ofcapillary, pulsation of test solution into the mouth of the capillarybetween drops, etc. It should be remembered in this connection that thereproducibility of the droplets with regard to their drop time and massof mercury per drop must be practically perfect at all times to permitproper evaluation of the polarogram.

Many attempts have been made to substitute better means for the droppingmercury electrode, such as multicapillaries, ejection of a stream ofmercury from the capillary into the test solution, bent electrodes toincrease drop size, forced dislodgment of droplets from the capillary inrapid succession, synchronization of mercury droplets being dropped frompairs of capillaries, oscilloscopio sweeps over successive drops atdefinite recurring time intervals, etc. Some of these methods arecumbersome to practice, some are extremely complicated if notimpracticable, and most have not produced more reliable data than theoriginal polarographic method, although in cases of special application,some benefit has been derived from these suggestions.

The present invention employs simple polarizable mercury electrodes inlieu of the classic dropping mercury electrode. Essentially, theelectrodes of this invention consist of a small amount of lmercury thatis separated from but electrolytically is put in communication with thetest solution by means of a thin porous wall of small dimensions.Preferably the mercury is held against the porous wall under lightpressure. In this arrangement, the test solution under investigationmigrates through the porous wall to be electro-deposited on the mercury.

The solid matrix of the porous wall must be chemically inert to both themercury and the test solution under investigation. It must not conductelectric current. The structure of said wall should be as open aspossible, i.e., the pore volume should -be as great as is consistentwith a sufficient structural rigidity to build and operate such anelectrode. The pores in said wall should be `small enough to prevent themercury from entering them, but should be large enough to permit thetest solution to diffuse freely through said porous wall to the mercury.In general, porous media with controlled porosities which are availablein commerce, such as microporous ceramic material with a pore volume ofabout 40% and an average pore diameter of about 5 to l0 microns, orfritted sintered glass with a similar degree of porosity and a similarpore size, are preferred in connection with pressures on the mercuryranging from about mm. Hg to 200 mm. Hg. The porous wall should have athin cross section to facilitate the migration of the test solution toand from the mercury; it should be just thick enough to have sufficientstructural strength for constructing and handling the electrode. Theabove mentioned microporous ceramic media may be formed into objects ofthe desired shapes with wall thicknesses of the order of one-sixteenthof an inch or even less.

The mercury in this electrode is preferably held against the porous wallunder slightpressure either derived from an elevated supply of mercuryconnected to it, or from a pump driving the mercury through theelectrode along the porous Wall, or by any other suitable means. Thispressure aids greatly in maintaining a coherent mercury surface at theboundary between the mercury and the porous wall, and thus itsubstantially eliminates migration of the test solution beyond theporous wall into that portion of the electrode that is assigned to themercury. n

The mercury may be kept stationary Within the electrode, provided it isfrequently renewed and provided the duration of the test runs is limitedto short periods, so as not to contaminate the mercury appreciably withdeposits of the electrode reaction. However, flow or streaming of themercury through the electrode along the porous wall is preferred,particularly in apparatus designed for continuous analysis, such as forprocess streams of the chemical industry. The ow of the mercuryfacilitates a continuous renewal of the mercury surface in the regionwhere the electrode process takes place.

The test solution contacting the mercury in the electrode via the porouswall may be stationary since the rate of migration of the test speciesto the mercury is generally greater than the depletion of the testspecies in the solution. The depletion is generally minimal, becausevery low current densities of the order of a few microamperes arenormally used to deposit the test species on the mercury. However,agitation and/or flow of the test soluvtion along the porous wall doesnot interfere ywith the analysis as would be the case with mostpolarographic cells employed heretofore. This feature is particularlyuseful in equipment facilitating the continuous analysis of anindustrial process stream or the like. i

The polarizable electrode of the present invention offers a number ofadvantages over the conventional dropping mercury electrodes, amongwhich are simplicity of design and operation, increased sensitivity,increased resolution of waves belonging to substances With'a similar[decomposition potential, substantial elimination of the condensercurrent, greatly increased -readability of polarograms due toelimination of oscillations in the. curves, minimization ofpolarographic maxima, and in 3 general ruggedness, since the electrodeof the invention is neither sensitive to mechanical vibrations, nor toagitation of the test solution during analysis, nor to small changes inthe height of the driving head of mercury.

In the accompanying drawings, two embodiments of the invention are shownby way of illustration. In a companion applicaton tiled lune 22, 1959,Serial No. 821,978, I have shown two additional polarographic cells, andhave presented broad claims which are intended to cover the four cellsof the two applications, as well as the method which is part of myinvention.

In said drawings,

FIG. 1 is a somewhat diagrammatic vertical section through apolarographic cell which is particularly adapted for the simultaneousdetermination of two test specimens in the same solution; v

FIG. 2 is a similar view of another form of polarographic cell which isparticularly useful for controlled potential separations.

Referring particularly to fthe drawings and iirst to FiG. 1, there isshown a polarographic cell including a mercury reservoir 11, a standtube 12 leading oilC from the bottom of reservoir 11, and a supply ofmercury 13 in reservoir 11 and stand tube 12. A second mercury reservoir11B, stand tube 12a, and mercury supply 13a are shown alongside of thecorresponding parts 11, 12 and 13. The lower ends of the stand tubes arebent to extend substantially horizontally, with the lower end of standtube 12a directly below the discharge end of stand tube 12. Theextremities 20, 2011 of lthe stand tubes are iixed to the lower portionof a vessel 32. The vessel 32 may be a generally cylindrical body ofglass or a resinous plastic such as epoxy resin, or any other suitablematerial, and it provides a container for the test solution underinvestigation, which is poured in at the top of the vessel. A centralbore 30 extends through the relatively massive bottom of vessel 32 andin this bore a microporous ceramic rod 34 is cemented, so that its upperend is in contact with the test solution 33. Microporous rod 34 may havea diameter of about onefourth of an inch, and an average pore diameterof about to 15 microns. The lower end of rod 34 extends into an opencontainer or beaker 13, in the bottom of which is a mercury poolelectrode 19. Lying on top of electrode 19 is an appropriateelectrolyte, such as an aliquot of the test solution 33. The microporousrod must be of such material that it is inert to mercury and the testsolution, and does not conduct electricity. With pores of the size rangementioned above, the mercury cannot enter the pores of rod 34 but thetest solution may migrate freely through the pores.

Small mercury-conducting passages 36, 36a run diametrically through thebottom of vessel 32 and registering passages 35, 35a are provided in themicroporous rod 34. Connection tubes 37, 37a are iitted in the bottom ofvessel 32 in alignment with passages 36, 36a respectively, and aresealed in position by rubber O-rings 3S, 38a. A sleeve 39 of rubbertubing couples tubes 37, 37a together. The common outlet tube 40 fortubes 37, 37a extends to a manually adjusted needle valve 41 having adischarge tube or outlet 42 for the mercury. A beaker or other vessel(not shown) will be placed be- 10W outlet 42 to collect the mercurydropping or streaming therefrom. Mercury supplies 13 and 1,3a andmercury pool 19 are connected by electrical conductors 58, 59 toapparatus (not shown) for determining of currentvoltage data in themanner well known in the art.

In operation, mercury from the supplies 13, 13a flows through therespective stand tubes 12, 12a, through passages 36, 36a, 35, 35-, tubes37, 37a and through ther needle valve 41 at a rate determined by `theopening of the needle valve. For example, the velocity of flow of themercury from the outlet 42 may be approximately one-half of one gram perminute. Test solution 33 slowly ows by gravity through microporous rod34 75 into vessel 18 forming electrolyte pool 33 therein.Electro-deposition of the test solution takes place at the mercurysurface formed by the mercury flowing through passages 35, 3521 in rod34. The mercury in each of the two reservoirs 11, 11a may be connectedto a predetermined applied potential, each in the manner well known inthe art, for determining single desired species in the test solution.Thus the test solution may be analyzed simultaneously for two species ofsubstance in the same test solution, by polarographic methods, whenusing the apparatus of FIG. l.

It is obvious that simultaneous analysis of several more sepcies of testsubstance in one test solution may be accomplished, if so desired, byconstructing a cell assembly of the type shown in FIG. l, but havingadditional passages through the microporous rod, means for ilowingmercury through such passages, and needle valve or other means for anice control of such ilow.

Now referring to the embodiment of the invention shown in FIG. 2, amercury reservoir 11 has a stand tube 12 leading olf from its bottomwall and holds a supply of mercury 13. The discharge end 20 of the standtube is sealed in the relatively massive bottom portion 45 of a vessel5t) which holds the test solution 51, poured in at the top. A smalltransverse bore 52 is provided in said bottom portion 45 to receive themercury ilowing from the stand tube and conduct it to a sm-all chamberS3 having cylindrical walls, located centrally (axially) of the bottomportion 45.

A microporous rod 54 is secured in said bottom portion to extend axiallythereof, the upper end of the microporous rod being in contact with thetest solution 51. Rod 54 extends axially through chamber 53 and projectsdownwardly from the bottom portion 45, its lower end being within anopen container or beaker 18. A mercury -pool electrode 19 is in thebottom of beaker 18, and lying on top of the electrode 19 is anappropriate electrolyte, such as an aliquot of the test solution 51. Rod54 is a ceramic solid rod about one-fourth of an inch in diameter, withan average pore diameter of about l() to 15 microns. Its material issuch that it is inert to mercury and to the test solution and it doesnot conduct electricity. With pores of the size range mentioned, Ithemercury in chamber 53 can not enter the pores, but the test solution maymigrate freely through the pores and ows down the rod by gravity fromthe vessel S0 to beaker 18.

As stated above, the transverse bore 52 delivers mercury to the centralchamber 53 at the bottom thereof. Another small transverse bore 55 isprovided in bottom portion 4S -at the -top of chamber 53 to conduct themercury away. A tube 56 receives the mercury from bore 5S and conductsit to a needle valve 41, thence to a discharge tube or outlet 4Z. Abeaker or other vessel (not shown) will be placed below outlet 42 tocollect the mercury dropping or streaming therefrom. The llow of themercury will obviously be governed by the degree of opening of theneedle valve. The velocity of ilow of the mercury may be adjusted by theneedle valve to approximately one-half of one gram per minute. Themercury supply 13 and the mercury pool electrode 19 are connected bysuitable conductors 6i), 61 to apparatus for determining ofcurrent-voltage data in the manner well Aknown in the art ofpolarography.

In operation, as the test solution 51 slowly descends microporous rod54, the test solution is plated out with respect to the desired testspecies, this test specimen being deposited on the mercury in chamber S3adjacent rod 54. The current derived from this unit is a direct measureof the concentration of the test species to be analyzed. The' embodimentof FIG. 2 permits one to plate out the desired test `species completely,which process is well known in the art as controlled potentialseparation. In continuous analysis, such as for instance with industrialprocess streams, any change of concentraaorsfrss tion of .theinvestigated species in the test solution will be indicated by a changein the current derived from this cell. Due to the relatively largesurface area of the mercury in chamber 53 and `therefore the relativelylarge currents obtained from even minute quantities of test species, theform of FIG. 2 is particularly valuable in the analysis of tracematerials.

1t will be obvious to those skilled in the art that the cells shown inFIGS. l and 2 may be combined to form an apparatus that would be veryuseful for the simultaneous separation and determination of complex testsolutions in a single operation combining controlled potentialseparation and polarographic current-voltage analysis. Other changes inthe apparatus will be obvious to those skilled in the art ofpolarography.

Having described two of the many forms which my invention may take, whatI claim as new and desire to secure by Letters Patent is:

1. Electrochemical apparatus comprising, in combination, a vessel havinga chamber adapted to contain a quantity of a test solution underinvestigation; a microporous rod whose upper end is adapted to be incontact with the test solution in said chamber; the lower end of saidrod being below the bottom of the chamber; said rod being of a materialthat is inert to mercury and to the test solution and that isnon-conductive of electricity; a source of mercury; means conducting themercury under low pressure to a wall of said rod which is also adaptedto be contacted by said test solution; the pores of said rod being largeenough and numerous enough to permit gravity ow of the test solutionfrom said chamber length- Wise of said rod, but said pores precludingthe mercury from entering them; means for conducting the mercury awayfrom said rod wall so that there may be a continuous ow of mercury pastsaid rod wall; a suitable electrode adapted to be in electrolyticcontact with said test solution below the lower end of said rod; andmeans for applying an electrical potential to said mercury versus saidelectrode, so as to produce a desired electrode reaction between thetest solution andthe mercury near said rod wall.

2. The invention dened in claim 1, wherein the microporous rod is asolid ceramic rod about one-quarter of an inch in diameter having anaverage pore diameter of about l0 to 15 microns.

3. The invention defined in claim 1, wherein the means for conductingmercury away from said rod is coupled to a needle valve which ismanually controllable to regulate ilow of mercury past said wall of saidrod.

4. The invention defined in claim l, wherein there is a containersurrounding the lower end of said microporous rod, a mercury poolelectrode being in the bottom of said container, and an appropriateelectrolyte lying on top of said mercury pool electrode and contactingthe lower end of said rod.

5. The invention defined in claim 1, wherein the microporous rod has atransverse passage therethrough; ther means for conducting mercury tosaid rod -discharges the mercury into said transverse passage; and themeans for conducting mercury away from said rod receives the mercurythat has owed through said transverse passage.

6. The invention defined in claim 5, wherein there is a manuallycontrolled needle valve to which the mercury is led by said means forconducting mercury away from said rod.

7. The invention dened in claim 1, wherein there are a plurality ofseparate sources of mercury, a plurality of transverse passages in themicroporous rod, a plurality of separate means for conducting mercuryunder low pressure to the respective transverse passages, and means forconducting away the mercury that has traversed said transverse passages.

8. The invention dened in claim 7, wherein there is a manuallycontrolled needle valve coupled to said means for conducting away themercury that has traversed said transverse passages.

9. The invention defined in claim 7, wherein the separate sources ofmercury are reservoirs which are elevated relative to the transversepassages in the microporous rod so as' to insure gravity tlow of themercury through said passages.

l0. The invention defined in claim 1, wherein the vessel has a mercurychamber formed therein and surrounding the microporous rod and having noconnection with the test solution chamber; the means for conductingmercury to said-rod discharges the mercury into said mercury chamber;and the means for conducting the mercury away from said rod conducts themercury from said mercury chamber.

11. The invention defined in claim 10, wherein there is a manuallyconrtolled needle valve to which the mercury is conducted by said meansfor conducting mercury away from said rod. v

12. The invention defined in claim 10, wherein there is a containersurrounding the lower end of said microporous rod; a mercury poolelectrode being in the bottom of said container; and an appropriateelectrolyte lying on top of said mercury pool electrode and contactingthe lower end of said rod.

13. An electrochemical method characterized by the step of causing atest solution under investigation to flow slowly through the pores of amicroporous body; said body being a non-conductor of electricity andbeing inert to mercury and to the test solution; the pores of said bodybeing small enough to prevent the mercury from entering; simultaneouslycausing mercury to ow under slight pressure along a wall of said bodywhich is contacted by said test solution, and applying an electricalpotential to the mercury via aA suitable electrode which is adapted tobe in electrolytic contact withthe test solution, so as to produce adesired electrode reaction between the test solution and the mercury.

14. An electrochemical method characterized by the step of causing atest solution under investigation to flow slowly through the pores of amicroporous rod; the rod being a non-conductor of electricity and beinginert to mercury and to the test solution; the pores of said rod beingsmall enough to precludev the mercury from entering; simultaneouslycausing mercury to flow along a passage inside said rod; and applying anelectrical potential to the mercury via a suitable electrode which isadapted to be in electrolytic contact with the test solution, so as toproduce a desired electrode reaction between the test solution and themercury owing along the passage inside said rod.

15. An electrochemical method characterized by the step of causing atest solution under investigation to flow slowly through the pores of amicroporous rod; the rod being a non-conductor of electricity and beinginert to mercury and to the test solution; the pores of the rod beingsmall enough to preclude the mercury from entering; simultaneouslycausing mercury to ow around a portion of the exterior of said rod, andapplying an electrical potential to themercury via a suitable electrodewhichy is adapted to be in electrolytic contact with the test solution,so that the test solution is plated out with respect to the desired testspecies, the test specimen beingv electro-deposited on the mercurysurrounding the rod.

y References Cited in the le of this patent UNITED STATES PATENTS

13. AN ELECTROCHEMICAL METHOD CHARACTERIZED BY THE STEP OF CAUSING ATEST SOLUTION UNDER INVESTIGATION TO FLOW SLOWLY THROUGH THE PORES OF AMICROPOROUS BODY; SAID BODY BEING A NON-CONDUCTOR OF ELECTIRCITY ANDBEING PART TO MERCURY AND TO THE TEST SOLUTION; THE PORES OF SAID BODYBEING SMALL ENOUGH TO PREVENT THE MERCURY FROM ENTERING; SIMULTANEOUSLYCAUSING MERCURY TO FLOW UNDER SLIGHT PRESSURE ALONG A WALL OF SAID BODYWHICH IS CONTACTED BY SAID TEST SOLUTION, AND APPLYING AN ELECTRICALPOTENTIAL TO THE MERCURY VIA A SUITABLE ELECTORDE WHICH IS ADAPTED TO BEIN ELECTROLYTIC CONTACT WITH THE TEST SOLUTION, SO AS TO PRODUCE ADESIRED ELECTRODE REACTION BETWEEN THE TEST SOLUTION AND THE MERCURY.